The present invention relates to laser devices in general and, more particularly, to laser configurations exhibiting narrow linewidth, low frequency chirp and broad wavelength tunability.
Single longitudinal mode laser sources with narrow linewidth, low frequency chirping and broad wavelength tunability are the essential components in optical communication systems, sensing systems and spectroscopy.
To achieve single longitudinal mode operation, it is widely recognized that some forms of filtering structure must be introduced in the laser cavity. The main approaches are the distributed feedback (DFB) laser [H. Kogelnik and C. V. Shank, xe2x80x9cCoupled-eave theory of distributed feedback lasersxe2x80x9d, Journal of Applied Physics, vol. 43, no. 5, pp. 2327-2335, 1972] and the distributed Bragg reflector (DBR) laser [S. Wang, xe2x80x9cPrincipals of distributed feedback and distributed Bragg-reflector waveguidesxe2x80x9d, IEEE Journal of Quantum Electronics, vol. QE-10, pp. 413-427, 1974.]. In both structures, a grating was embedded in the laser cavity with or without gain in the grating regions. Compared to conventional Fabry-Perot lasers, grating-based DFB and DBR lasers need much more complicated material growth and processes and add considerable cost.
Once single longitudinal mode operation is achieved, the laser linewidth needs to be as narrow as possible to reduce the phase noise. It is well known that increasing the photon lifetime is the most effective way to reduce the linewidth. Generally, this could be achieved by extending the laser effective cavity length. However, the physical cavity length is inherently limited in monolithic DFB and DBR lasers. Therefore, to achieve a very narrow linewidth in DFB and DBR lasers is difficult. Another approach is external cavity lasers, which allow a dramatic cavity length increase and yield very narrow linewidth [R. Wyatt, xe2x80x9cSpectral linewidth of external cavity semiconductor lasers with strong, frequency-selective feedback,xe2x80x9d Electronics Letters, Vol. 21, pp. 658-659, 1985.]. But the bulky device size, the complicate package and the high cost of external cavity lasers limit their applications.
Wide wavelength tunability is necessary for many applications. The lasing wavelength in standard DFB and DBR lasers can be tuned by changing the refractive indices of different layers. But this tuning is limited to a small range due to the small change of the refractive index with electro-optic, electro-absorption, or thermo-optic effects. To leveraging the wavelength tunability, sampled gratings [V. Jayaraman, Z. M. Chuang, and L. A. Coldren, xe2x80x9cTheory, Design, and Performance of extended tuning range semiconductor lasers with sampled gratings,xe2x80x9d IEEE Journal Quantum Electronics, Vol. QE-29, pp. 1817-1823, 1993.] have been introduced to DBR lasers. By using Vernier effect in two sampled gratings, the wavelength tuning range can be enhanced a factor of xcx9c10 with the maximum index change xcx9c1% by injection carriers (electrons or holes) in different layers. Carrier injection is attractive for its large index change, but it also has several disadvantages. First, the carrier injection causes the internal optical loss, which affects the uniformity of lasing threshold currents and the electron to photon efficiencies at different wavelengths. Second, the switching time via carrier injection is in the range of tens of nanoseconds, not fast enough for some applications. For ultrafast tuning speed, electrooptic (EO) effect is preferred which has no induced optical loss. But the index change by EO effect is much smaller than the change by carrier injection. Thus, a large tuning enhancement is needed to use the EO effect for the tuning purpose.
Micro-ring lasers [J. P. Hohimer, D. C. Craft, G. R. Hadley, G. A. Vawter and M. E. Warren, xe2x80x9cSingle-frequency continuous-wave operation of ring resonator diode lasers,xe2x80x9d Applied Physics Letters, Vol. 59, pp. 3360-3362, 1991.] were thought to be another laser structure for single longitudinal operation. But conventional ring lasers, where the active ring resonator replaces the standing wave Fabry-Perot cavity, have the same limitations to achieve narrow linewidth and wide wavelength tunability as Fabry-Perot lasers due to the limited cavity length and the index change. The ring laser has another disadvantage of the inherent unstability caused by the mode competition between the clockwise and anti-clockwise propagation traveling-waves.
It is an object of the present invention to provide a laser arrangement which operates at a single longitudinal mode with a high side mode suppression ratio and without the grating mode selection filter in the aforementioned laser arrangements.
It is another object of the present invention to provide a laser source that exhibits very narrow linewidth and low frequency chirping.
It is still another object of the present invention to provide a laser source which has a wide wavelength tuning range.
It is yet a further object of the present invention to provide a laser source which has a very fast wavelength tuning speed.
Other objects and advantages of the present invention will become apparent from a consideration of the following detailed description with reference to the accompanying drawings.
The foregoing objects have been achieved by a passive ring resonator coupled laser of the present invention comprising passive micro-ring resonator, active gain section, a pair of reflective mirrors and passive optical waveguides connecting the reflective mirrors, the ring resonator and the gain section.
A preferred embodiment of the ring resonator coupled laser according to the invention is characterized in that one passive ring resonator is interposed between one of the two mirrors and the gain section and connected by the two passive waveguides. The passive ring resonator acts as a strong mode selector for obtaining a single longitudinal mode of operation over a very narrow frequency linewidth with a very high side mode suppression ratio.
Another preferred embodiment of the passive ring resonator coupled laser according to the invention is characterized in that a pair of ring resonators are interposed between the two mirrors and the gain section, and connected by the passive waveguides. The two ring resonators have two sets of transmission spectra with slightly different wavelength peak spacing. By adjusting the refractive index of the one or two ring resonators, one of the peaks in the two sets of spectra can be chosen to match to a desired wavelength and a wide wavelength tuning range of the laser can be achieved. Electro-optic effect is preferred as a mean of adjusting the refractive index for high speed wavelength tuning, but not the only possible mean.